It is often necessary to convert power provided by a voltage source at a first DC voltage into power at a different DC voltage; this is typically done by DC-DC converters. DC-DC converters are commonly used in battery chargers, radios, television receivers, computers, cell phones and other devices. It is desirable that these converters operate with high efficiency. It is also often desirable that these converters be low in cost, since they are often manufactured in very high volumes.
A common DC-DC converter architecture, as illustrate in FIG. 1, is the buck converter. These converters typically operate as a step-down converter where a high input voltage 102 is coupled by switching device 104 to inductor 106 during a first interval, both storing energy in magnetic fields of inductor 106 and providing current to an output 110 and an output filtering capacitor 112; then in a second interval switching device 104 turns off and current continues to flow in inductor 106 and a second switching device or diode 108 while the magnetic fields of inductor 106 decay. Filtering capacitor 112 helps level the output 110 voltage, and provides power to the load during any interval in which inductor 106 is not conducting current.
Another common DC-DC converter architecture, as illustrated in FIG. 2, is the boost converter. These operate by closing a switching device 134 to build up a current and store energy in an inductor 136 from an input power supply 132 during a first interval, then in a second interval opening switching device 134 and drawing energy from the inductor 136 through a second switching device or diode 138 to drive a load 140. A capacitor 142 may be provided as a filter that provides energy to the load when device 138 is not conducting.
Yet another common DC-DC converter architecture, as illustrated in FIG. 2A is the buck-boost converter. These operate by closing a switching device 145 to build up a current in inductor 147 from an input power supply 143 during a first interval, then in a second interval opening switching device 145 and drawing energy from the inductor 147 through a second switching device or diode 149 into a filtering capacitor 151 and load 153.
Buck, boost, and buck-boost converters provide an output power having a ripple component which must be filtered by capacitors 112, 142, 151. In a buck converter as illustrated in FIG. 1, inductor 106 operates to store energy while switching device 104 conducts, and to release energy during at least part of the time switching device 104 is off, similarly the boost converter of FIG. 2 stores energy in inductor 136 when switching device 134 conducts, and releases energy during at least part of the time switching device 134 is off, and the buck-boost converter of FIG. 2A stores energy in inductor 147 when switching device 145 conducts and releases that energy during at least part of the time switching device 145 is off; inductors 147, 136 and 106 are therefore the primary energy storage inductors of these converters. Additional inductors may be present in buck, boost, and buck-boost configurations for such purposes as filtering on input and output terminals, and other purposes.
When large output currents are required, it is common to couple multiple DC-DC converter units, such as the buck converter of FIG. 1, in parallel having a common input 102 and output 110A, such as shown in FIG. 3. Since the capacitance required for reducing ripple by filtering is inversely proportional to a frequency of the ripple component, operation of switching devices, such as switching devices 104, 108, of each parallel-connected converters 160, 162 are typically offset in time from one converter 160 to another 162, this results in effectively multiplying a frequency of the ripple component by the number of parallel connected converters and reducing required capacitance of filter capacitors 112. Because of the time-offset, each converter 160, 162, of such a parallel arrangement of DC-DC converters is known as a phase of the overall converter system.
It is known that efficiency of parallel DC-DC converter units is enhanced, and ripple reduced, if there is a degree of magnetic coupling between the inductor 106A (representing primary energy storage inductor 106 of a first component buck converter similar to that of FIG. 1) of one phase 160, with the inductor 106B (representing primary energy storage inductor 106 of another component buck converter similar to that of FIG. 1) of a second phase 162 having timing offset from that of the first phase. Such an arrangement appears in U.S. Pat. No. 6,362,986 to Schultz et al. (Schultz), which is incorporated herein by reference. Typically, such a converter arrangement also has a common control circuit 164 that controls all phases, such as phases 160, 162, to provide proper drive to the output 110A. Since the output voltage of all of the parallel-coupled converters is the same, these converters are not independent converters.
It is also known that perfect coupling between primary energy storage inductors of different phases of paralleled DC-DC converters is undesirable. In various embodiments, these converters may use a transformer formed of coupled inductors having substantial leakage inductance, or a transformer with a separate inductor in series with each winding of the transformer; the term magnetically coupled inductors in this document shall include both configurations.
DC-DC converters having large differences between input and output voltages can potentially have improved efficiency if they include a first-stage converter that converts an input voltage to an intermediate voltage, and a second-stage converter that converts the intermediate voltage to the required output voltage. H. Nakanishi, et al., in A Two-stage Converter with a Coupled-Inductor, 7th International Conference on Power Electronics and Drive Systems, 2007, 653-657 (PEDS '07), reported an analysis of a two-stage buck-converter arrangement similar to that illustrated in FIG. 4. In the arrangement of PEDS '07, a first buck converter 180 having inductor 184 reduces an input voltage 182 to an intermediate voltage 186. A second buck converter 188 having inductor 190 reduces the intermediate voltage to output voltage 192. Inductors 184 and 190 are magnetically coupled—PEDS '07 shows each inductor winding as having an ideal transformer portion and an inductor portion. The circuit analyzed in PEDS '07 is limited, however, to one input 182 and one output 192, and does not disclose separate feedback control loops for the converter.
A converter as illustrated in FIG. 3 has a single input and a single output voltage. There are many applications where more than one output voltage is desired in a system; for example a common personal computer may operate a processor core at 1.5 volts, a processor periphery and memory at 3.3 volts, other logic at 5.0 volts, disk motors at 12.0 volts, and other circuitry at −5.0 volts. It is desirable to provide regulated outputs at more than one output voltage to enable operation of such systems.